If you're browsing in a private-window, this means the browser cannot remember (save in a cookie) that you clicked "close" or saw the banner already, hence you might be seeing more banners than most readers would.
Worth mentioning each time it comes up: This seems like an instance of where tracking interactions w/ a given {IP, day} or {hash of browser fingerprint+IP+daily salt} would vastly outweigh the challenges. We can support private browsing, or any kind of browsing, without creating a painful experience [which is also... not likely to result in a donation].